This invention relates to a method and apparatus for twisting insulated conductors for use in multiconductor communication cables to form star quads, and more particularly to a method and apparatus for twisting four insulated conductors for use in multiconductor communication cables into star quads by pretwisting the insulated conductors.
Various methods of twisting insulated conductors for use in multiconductor communication cables into quads have been proposed in the past. One prior art method comprises the steps of mounting four supply bobbins of insulated conductors for use in multiconductor communication cables on a turn table driven by an electric motor, pretwisting the insulated cnductors paid off from the supply bobbins by means of an insulated conductors pay-off member which is used to drive a turn table by an electric motor when the insulated conductors are paid off, and then supplying the pretwisted insulated conductors to a twisting member. According to another prior art method four sets of insulated conductor pay-off members are provided each comprising supply bobbins and a rotary flyer which are mounted over a stationary floating table such thaat the insulated conductors are paid off from the supply bobbins via the rotary flyers and the rotary flyer is rotated about the supply bobbins on the stationary floating tables so as to pretwist twice times the insulated conductors paid off from respective supply bobbins. Then the pretwisted insulated conductors are sent to the twisting member to form a quad.
According to these methods, since the supply bobbins are rotated or the rotary flyer is rotatably mounted over the floating table for the purpose of providing pretwists to respective insulated conductors the mechanism for rotating the bobbins or rotary flyer is complicated thus limiting the rotating speed. Assuming now the V represent the running speed of respective insulated conductors and that P the pitch of the twist of quad, the revolutions N of the supply bobbin per unit time will be obtained by the expression N=.sup.V/ P. It is understood from this expression that running speed V of the insulated conductors is NP, i.e. V=NP, and it depends on the number of revolution N and pitch P.
Pretwists are applied to the insulated conductors for the following reason. In a multiconductor communication cable comprising an assembly of a plurality of star quads each formed by twisting four insulated conductors it is necessary to reduce crosstalks caused by the capacitance unbalance between two pairs in a star quad. Especially, for the purpose of decreasing the capacitance unbalance between two pairs in a star quad it is advantageous to position respective insulated conductors on the apices of a square or a rhomb lying in the cross-section of the star quad. However, since the insulated conductors for use in multiconductor communication cables are generally prepared by extruding plastic coatings on cnductors by an extruder it is difficult to accurately position the conductors at the centers of the insulative coatings, in other words, to make the conductors and the insulative coatings to be in exact concentric relation. Thus, in an insulated conductor prepared in this manner, the conductor is slightly eccentric with respect to the center of the insulative coating, such eccentric condition continuing along the length of the insulated conductor. Where polyethylene is used as the insulation the plastic is often foamed to decrease the dielectric constant thereof. However, foaming is not uniform in the cross-section of the insulated conductor depending upon the conditions of the extruder. Sometimes, non-uniform foaming continues along the length of the insulated conductor. In this manner, where the conductor is eccentric with respect to the insulative coating or where the formed plastic coating contains nonuniformly distributed foams, even when four such deficient insulated conductors are twisted into a star quad in such a manner that respective insulated conductors will be positioned exactly on the apices of a square or a rhomb, the insulated conductors would not be positioned correctly on the apices of the square or the rhomb, or in certain cases, the dielectric constant between respective conductors would not be uniform. This causes an unbalance in the mutual capacitances between four pairs in a star quad. In other words, two circuits comprising a pair of conductors in a quad will be electrostatically coupled together and thus deteriorate the crosstalk characteristic of the cable.
To solve these problems a method of twisting in which pretwists are applied to the insulated conductors has been proposed. With this method even when the conductor is eccentric with respect to the plastic coating along the length of the insulated conductor or the formed insulative conductor is not uniform, as the positions of respective insulated conductors which face each other vary the eccentric condition of the conductors also varies along the length of the insulated conductors thus making more uniform the mutual capacitance between respective conductors with the result that the crosstalk characteristic is greatly improved than in a star quad without pretwist.
It has been accepted that in a twisted quad the pitch of the pretwist which is necessary to minimize the capacitance unbalance should be equal to the pitch of the quad. In recent years, for the purpose of improving the efficiency of twisting the insulated conductors it has been proposed to increase the size of the supply bobbins and the number of revolutions of the twisting apparatus. This renders it difficult to form a required number of pretwists to the paid off insulated conductor so that it has been obliged to sacrifice the production speed for giving pretwists of the desired member.
Further, in the prior art method of twisting insulated conductors into a quad as described above, it is necessary to rotate the supply bobbins of the insulated conductors and to surround the insulated conductor pay off bobbins with a rotary flyer. Hence, it has been impossible to exchange automatically the bobbins and to continuously pay off the insulated conductors.